1. Field of the Invention
The present invention relates to a high dispersible hydrophobic fine silica powder and a production method thereof. The high dispersible hydrophobic fine silica powder of the present invention is suitable as a filler for controlling a viscosity for resins, such as adhesives, coating materials or the like, a reinforcing filler for natural rubber, synthetic rubber, engineering plastics or the like, a filler for an antiblocking agent for films of polyethylene, polyester or the like, or a flowability improving agent for toners for electrophotography, powder coating materials or the like.
2. Description of the Prior Art
It has been known that a silica powder can have a multifunction property, by surface modifying with silane coupling agents or surface modifying agents, such as polymer or the like, according to its use and chemically changing hydroxyl groups on the powder surface. For example, it has been carried out that a silica powder generally having hydrophilicity with the hydroxyl groups on the powder surface is changed to the hydrophobic silica powder by using suitable coupling agents or polymer. More concretely, the hydrophobic silica carried out a hydrophobic treatment with an organic halosilane has been publicly known conventionally, and this silica is described, for example, in the specification of West German Patent No. 63784.
The hydrophobic fine silica powder is used as the filler and the reinforcing material of resins, the fluidizing agent of a resin powder, or the like. By the hydrophobic treatment, when the fine silica powder is added to resins, such as silicone resins, epoxy resins or the like, it is possible to increase the dispersibility of the resin, and also increase a mechanical strength of the hardened materials. Further, it is possible to obtain the effects, such as increasing the flowability of the resin powder, preventing the adsorption of moisture, or the like. Especially, it has been known that the fine silica powder having a large specific surface area is effective to increase the flowability of the resin powder since it has a small average primary particle size, and to have a high reinforcing effect as the filler for resins, rubbers or plastics. Further, in the case of a transparent resin or the like, it has been known that the resin has a high transmittance in its transparency.
As for the conventional hydrophobic silica powder, the powder having the comparatively small specific surface area, more concretely, having the specific surface area of less than 200 m2/g can have the uniform hydrophobicity. However, the hydrophobic treated silica powder having the specific surface area of more than 200 m2/g, has problems mostly that the uniformities of the hydrophobicity and the treatment are low. In this case, even when the amount of the hydrophobic agent is increased in order to increase the treating effect, it is difficult to obtain the uniformly hydrophobic silica powder since the fine silica powder having the large specific surface area has a strong agglomeration. So, it has been provided that a gas flow rate in the fluidized bed is controlled in the range of from 1.4 to 3.0 cm/sec by controlling the flowing rate to prevent the scattering of silica. By this way, the amount of the treating agent can be decreased without decreasing the hydrophobic treating level. (Japanese Patent Laid Open No. 2001-261327) However, this silica powder is not the fine silica powder having the large specific surface area provided in the present invention, but the powder is the grade having about 130 m2/g of the specific surface area.
Further, when the reaction temperature is increased in order to increase the treating efficiency, there is a problem that an organic group of the treating agent or on the surface of the silica powder is decomposed. When the organic group on the surface of the silica powder is decomposed, the hydrophobicity is not only bad influenced but also the property of the hydrophobic silica powder is influenced so much. For example, although triboelectrostatic charge is obtained by the surface contact with an iron powder, the objective triboelectrostatic charge cannot be obtained when the organic group on the powder surface is decomposed. Further, when the organic group on the powder surface is decomposed, the fine silica powder may be colored to yellowish or the like, with the generated carbon. Such yellowish color damages the appearance of the resins at the time of adding to silicone resins, acrylic resins or the like. Further, an electrostatic chargeability of the silica powder is decreased due to the conductivity of the generated carbon. Therefore, when said silica powder is used as an external additive for toners, powder coating materials or the like, the electric properties, such as the electrostatic chargeability, are damaged.
On the other hand, when the reaction time, that is, the staying time in reaction gas becomes long without increasing the reaction temperature, the productivity decreases. Moreover, in the case of using the fluidized bed type reaction vessel, when the staying time becomes long, the silica powder is easily agglomerated to decrease the dispersibility, thus the triboelectrostatic charge is bad influenced. Further, since the dispersibility is decreased, the transparency at the time of kneading with the resin is also damaged.